1. Field of the Invention
This invention relates to imaging of the human iris, as may be used for biometric identification. More specifically, this invention relates to post processing of images of the human iris, as may be used to increase the quality of these images.
2. Description of the Related Art
As traditional forms of personal identification become vulnerable to advancing technology, biometric identification is increasingly seen as a viable approach to personal identification. Techniques such as voice recognition, fingerprinting, and iris imaging rely on physical personal traits that are difficult to change or duplicate.
However, biometric identification via iris imaging typically requires a high resolution image of the iris in order to resolve the fine details necessary to make a positive identification. An image of an iris with approximately 200 micron or better spatial resolution typically is required to uniquely distinguish the fine muscle structure of human irises, as may be required for identification purposes. In systems where the subject is actively cooperating, conditions such as illumination geometry, camera resolution, exposure time, and wavelength of light can be optimized in order to capture a high contrast and high resolution image of the fine structure of the iris. However, the situation becomes significantly worse when the subject is not actively cooperating.
The “capture volume” of an iris imaging system is the volume over which the iris imaging system can capture iris images of sufficiently high resolution. Traditional systems have a small capture volume—so small as to make traditional iris imaging systems unsuitable for use in uncooperative situations, such as iris imaging over large groups of people, over longer standoff distances, or for covert identification applications. For example, it may be desirable to capture iris images of subjects as they walk through a portal, such as a metal detector, or in places like airports, train stations, border crossings, secure building entrances and the like.
However, the high resolution and longer standoff requirements in these applications place significant challenges on iris imaging systems. For example, a short standoff system using commercial CCD technology (e.g., 5 megapixels) could have a field of view of approximately 15 cm at a 1 m standoff range, yielding a spatial resolution of approximately 75 microns per pixel at the 1 m standoff range. This resolution is sufficient for biometric identification. However, a 1 m standoff range typically will require the subject's active cooperation to position their head in the field of view and at the correct focus distance.
If the standoff range is increased to 10 m for example, the situation becomes significantly more difficult. If the same camera were used at a standoff of 10 m, maintaining the same angular resolution would result in a spatial resolution of 750 μm per pixel, which is not sufficient for biometric identification. The imaging optics could be designed to yield a spatial resolution of 75 μm per pixel, but this would then result in a 15 cm wide field of view at 10 m. Keeping the iris within this field of view is difficult. The field of view could be increased while maintaining the same resolution by increasing the number of pixels in the camera, but this increases the cost and complexity of the camera.
Therefore, there is a need for iris imaging systems capable of generating sufficiently high resolution and high quality iris images, and at longer standoff distances and with sufficiently large fields of view but without requiring unusually large or complex cameras. These systems could be used to enable iris imaging over increased capture volumes and/or at longer standoff distances.